fix: preserve whitespace-only chunks in SSE stream (#139)

* fix: preserve whitespace-only chunks in SSE stream

Fixes issue #138 where Claude models would swallow spaces between words because whitespace-only chunks (e.g., " ") were being filtered out as empty.

Changes:
- Modified sse-streamer.js to only skip truly empty strings (""), preserving strings that contain only whitespace.
- Added regression test case in tests/test-streaming-whitespace.cjs to verify whitespace preservation.

* test: add streaming whitespace regression test to main suite

---------

Co-authored-by: walczak <walczak@ial.ruhr>

src/cloudcode/sse-streamer.js

@@ -123,8 +123,8 @@ export async function* streamSSEResponse(response, originalModel) {
                         };
 
                     } else if (part.text !== undefined) {
-                        // Skip empty text parts
-                        if (!part.text || part.text.trim().length === 0) {
+                        // Skip empty text parts (but preserve whitespace-only chunks for proper spacing)
+                        if (part.text === '') {
                             continue;
                         }